Fabrication of highly efficient TiO2/Ag/TiO2 multilayer transparent conducting electrode with N ion implantation for optoelectronic applications

Fabrication of highly conductive and transparent TiO₂/Ag/TiO₂ (referred hereafter as TAT) multilayer films with nitrogen implantation is reported. In the present work, TAT films were fabricated with a total thickness of 100 nm by sputtering on glass substrates at room temperature. The as-deposited films were implanted with 40 keV N ions for different fluences (1×10¹⁴, 5×10¹⁴, 1×10¹⁵, 5×10¹⁵ and 1×10¹⁶ ions/cm²). The objective of this study was to investigate the effect of N⁺ implantation on the optical and electrical properties of TAT multilayer films. X-ray diffraction of TAT films shows an amorphous TiO₂ film with a crystalline peak assigned to Ag (111) diffraction plane. The surface morphology studied by atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM) revealed smooth and uniform top layer of the sandwich structure. The surface roughness of pristine film was 1.7 nm which increases to 2.34 nm on implantation for 1×10¹⁴ ions/cm² fluence. Beyond this fluence, the roughness decreases. The oxide/metal/oxide structure exhibits an average transmittance ~80% for pristine and ~70% for the implanted film at fluence of 1×10¹⁶ ions/cm² in the visible region. The electrical resistivity of the pristine sample was obtained as 2.04×10⁻⁴ Ω cm which is minimized to 9.62×10⁻⁵ Ω cm at highest fluence. Sheet resistance of TAT films decreased from 20.4 to 9.62 Ω/□ with an increase in fluence. Electrical and optical parameters such as carrier concentration, carrier mobility, absorption coefficient, band gap, refractive index and extinction coefficient have been calculated for the pristine and implanted films to assess the performance of films. The TAT multilayer film with fluence of 1×10¹⁶ ions/cm² showed maximum Haacke figure of merit (FOM) of 5.7×10⁻³ Ω⁻¹. X-ray photoelectron spectroscopy (XPS) analysis of N 1s and Ti 2p spectra revealed that substitutional implantation of nitrogen into the TiO₂ lattice added new electronic states just above the valence band which is responsible for the narrowing of band gap resulting in the enhancement in electrical conductivity. This study reports that fabrication of multilayer transparent conducting electrode with nitrogen implantation that exhibits superior electrical and optical properties and hence can be an alternative to indium tin oxide (ITO) for futuristic TCE applications in optoelectronic devices.     

Performance optimization of fluorine-doped tin oxide thin films by introducing ultrasonic vibration during laser annealing

Commercial fluorine-doped tin oxide (FTO) thin films were subjected to laser annealing coupled with ultrasonic vibration (48 kHz and 350 W). The effects of ultrasonic vibration, laser fluence and defocusing amount were systematically studied. Laser annealing could result in grain growth or damage of the FTO layer, and introducing ultrasonic vibration during laser annealing could effectively enhance the film compactness, decrease the film thickness and refine the grains in the film. As a result, the optical and electrical properties of the ultrasonic-vibration-assisted laser-annealed FTO films were significantly improved by using low laser fluences and high defocusing amounts, and were slightly deteriorated when high laser fluences and low defocusing amounts were adopted. The results indicated that the film obtained by ultrasonic-vibration-assisted laser annealing using a laser fluence of 0.6 J/cm² and a defocusing amount of 2.0 mm had the best overall photoelectric property with an average transmittance of 84.1%, a sheet resistance of 8.9 Ω/sq and a figure of merit of 1.99×10^–2 Ω^–1, outperforming that of the film obtained by pure laser annealing using the same experimental parameters. The present study confirms the efficacy of ultrasonic-vibration-assisted laser annealing in optimizing performance of FTO films.     

Densification and properties of transition metal borides-based cermets via spark plasma sintering

Engineering borides like TiB₂ and ZrB₂ are difficult to sinter materials due to strong covalent bonding, low self-diffusion coefficient and the presence of oxide layer on the powder particles. The present investigation reports the processing of hard, tough and electrically conductive transition metal borides (TiB₂ and ZrB₂) based cermets sintered with 6 wt.% Cu using spark plasma sintering (SPS) route. SPS experiments were carried out with a heating rate of 500 K/min in the temperature range of 1200–1500 °C for a varying holding time of 10–15 min and the optimization of the SPS conditions is established. A maximum density of ∼95% ρ_th in ZrB₂/Cu and ∼99% ρ_th in TiB₂/Cu is obtained after SPS processing at 1500 °C for 15 min. While the optimized TiB₂/Cu cermet exhibits hardness and fracture toughness of ∼17 GPa and ∼11 MPa m^1/2, respectively, the optimized ZrB₂/Cu cermet has higher hardness of ∼19 GPa and fracture toughness of ∼7.5 MPa m^1/2, respectively. High electrical conductivity of ∼0.20 MΩ⁻¹ cm⁻¹ (TiB₂/Cu) and ∼0.15 MΩ⁻¹ cm⁻¹ (ZrB₂/Cu) are also measured with the optimally sintered cermets.     

Excimer laser reduction and patterning of graphite oxide

A successful approach and the operational parameters necessary for reduction of graphite oxide (GO) to multilayer graphene using 248 nm excimer laser irradiation in both vacuum and ultrahigh purity N₂ background environments is described. The utility of excimer laser reduction is demonstrated by production of simple line and logo patterns using standard microscale lithographic patterning strategies. Multilayer graphene formation is confirmed with Raman and X-ray photoelectron spectroscopies, and the morphology of the processed GO sample is evaluated with scanning electron microscopy. Four-point probe measurements of the excimer laser reduced GO indicate typical sheet resistances of ∼100–500 Ω/sq, which is a significant improvement over other values reported in the literature for other laser-based GO reduction methods.     

Ion-induced electron emission (IIEE) from undoped and B-doped diamond films induced by 1–10 KeV H+ and Ar+

A newly developed experimental system enables measurements of IIEE at a very low ion flux (10⁵ ions/s cm²) avoiding the fast emission degradation caused by high ion fluxes (usually applied 10¹⁰–10¹³ ions/s cm²). The method overcomes the difficulty of measurement of reliable ion-induced secondary electron emission (IIEE) yield, associated with fast degradation of the IIEE yield. We report on the investigation of the IIEE from hydrogenated undoped and B-doped diamond films as a function of (i) moderate heating in vacuum prior to the measurements, (ii) H⁺ and Ar⁺ energy in the range of 1–10 KeV, and (iii) film thickness and microstructure. An IIEE yield (γ) enhancement was typically detected when the films were heated to 300 °C in vacuum. In the B-doped diamond film heated to 300 °C, γ rose nearly linearly from ∼ 20 to ∼ 100 electrons/ion, for 1 to 10 KeV Ar⁺ ions, respectively. The values of γ obtained with H⁺ showed a more moderate, nonlinear increase from ∼ 8 electrons/ion at 1 KeV up to ∼ 90 electrons/ion at 10 KeV. In heated undoped diamond films of different thicknesses the measured values of γ were similar for all the studied films and somewhat lower than in B-doped film: from ∼ 10–16 to 60–70 electrons per 1 to10 KeV Ar⁺, respectively, and from 14–26 to 50–60 electrons per 1 to10 KeV H⁺, respectively. The experimental results were interpreted using TRIM calculations.     

Direct multipulse laser processing of titanium oxide–graphene oxide nanocomposite thin films

Nanocomposite thin films consisting of titanium oxide (TiO₂) nanoparticles (NPs) and graphene oxide (GO) platelets were deposited by a spin-coating technique. The obtained films were submitted to direct laser irradiation using a frequency quadrupled Nd:YAG (λ=266 nm, τ_FWHM≅3 ns, ν=10 Hz) laser source. The effect of the laser processing conditions, as laser fluence value and number of subsequent laser pulses incident onto the same target location, on the surface morphology, crystalline structure, and chemical composition of the TiO₂/GO nanocomposite thin films was systematically investigated. The laser fluence values were maintained below the vaporization threshold of the irradiated composite material. With the increase of the laser fluence and number of incident laser pulses melting and coalescence of the TiO₂ NPs into inter-connected aggregates as well as rippling of the GO platelets take place. The gradual reduction of GO platelets and the onset of anatase to rutile phase transition were observed at high laser fluence values.     

Carbon nanotubes growth in AlPO4-5 zeolites: Evidence for density dependent field emission characteristics

We report on the correlation between the concentration of Fe-catalyst, doped in the aluminum phosphate (AlPO₄-5) zeolite and the resulting density of carbon nanotubes (CNTs) to obtain the optimum electron field emission conditions from the CNTs. Initially, AlPO₄-5 crystallites were impregnated, for a period of ∼ 10–60 min, in the Fe-catalyst solution and subjected to Electron Spectroscopy for Chemical Analysis (E.S.C.A.). The analysis revealed that the concentration of Fe-catalyst, C_Fe, was increased from ∼ 1.7% to ∼ 8.6%, respectively, with increase in impregnation time, I_T. The HRTEM results showed that Fe nano-clusters, with diameter ∼ 7–10 nm, were formed in the surface region of the crystallites. These crystallites were sprayed on the conducting substrates, under identical spraying conditions. SEM study revealed that the coverage of the crystallites on the substrates was ∼ 10³–10⁴ crystallites/cm². These substrates were subjected to direct current plasma enhanced chemical vapor deposition (dc-PECVD) process, to grow CNTs. The SEM micrographs were recorded for the CNT-grown substrates and the average areal density of CNTs, (σ_T)_av, on the crystallites (t/cm²) was estimated. The analysis indicated that (σ_T)_av increased from ∼ 6.24 ± 0.19 × 10¹⁰ to 2.04 ± 0.61 × 10¹¹ t/cm² with gradual increase in C_Fe. The field emission study of the samples revealed that the optimum values of the turn-on electric field, ∼ 3.69 V/μm and the field emission current density, ρ_d, ∼ 1.78 × 10³ μA/cm² were achieved for (σ_T)_av, ∼ 6.24 ± 0.19 × 10¹⁰ t/cm², at a concentration of Fe, C_Fe, ∼ 3.0%, encapsulated in the AlPO₄-5 crystallites.     

Spectroscopic evaluation of out-of-plane surface vibration bands from surface functionalization of graphite oxide by fluorination

The fluorination of graphite/graphite oxide (GO) and their derivatives has been widely investigated for how fluorine interacts with sp²/sp³ carbon; however, the mechanism of this interaction has not yet been elucidated. Fluorination of GO (FGO) at either 10 or 15 psi for 24 h, produced two new absorption bands at ∼743 cm⁻¹ and 482 cm⁻¹, and are attributed to the presence of out-of-plane surface fluorine bonds in FGO (absent in fluorographite – FG). IR studies confirmed the stability of the formed C–F bonds and defect formation due to the introduction of oxyfluorinated species into the graphitic carbon through fluorination of epoxides. Fluorination of GO resulted in ∼4–5 times more fluorine incorporation in bulk as compared to FG. (4.57 vs. 0.8 at.% and 6.64 vs. 1.4 at.% at 10 and 15 psi, respectively). PXRD analyses also showed that the interlayer spacing of FGO expanded in the presence of intercalated C–F species and a defect formation was observed with the evidence of increase of the I_D/I_G ratio from Raman spectra. To this end, understanding the origin of surface C–F bonds and structural changes in FGO therefore leads to new applications such as implementation of FGO for sensing, nano-electronics and energy storage.     

Hybrid carbon source for single-walled carbon nanotube synthesis by aerosol CVD method

The conductivity enhancement of single-walled carbon nanotube (SWCNT) films was achieved by increasing the bundle length in an aerosol CVD synthesis method with the help of two carbon sources. Carbon monoxide provides carbon at temperatures below 900 °C, while ethylene takes over at higher temperatures. The significant decrease in the sheet resistance at the 90% transmittance was observed from 3500 to 7500 Ω/sq. for pure CO system via 1909 and 1709 Ω/sq. for CO–H₂ system to 291 and 358 Ω/sq. in the presence of C₂H₄ at 900 and 1100 °C, respectively. Doping the film with a gold chloride solution in acetonitrile allowed us to create the transparent conductive films with the sheet resistance as low as 73 Ω/sq. at a transmittance of 90%.     

Preparation and characterization of transparent indium- doped TiO2 films deposited by MOCVD

Titanium dioxide (TiO₂) films doped with different indium (In) concentrations have been prepared on SrTiO₃ (STO) substrates by high vacuum metalorganic chemical vapor deposition (MOCVD). X-ray diffraction (XRD) analyses revealed the TiO₂ films doped with low In concentrations to be [001] oriented anatase phase and the films with high In concentrations to present polycrystalline structures. The 1.8% In-doped TiO₂ film exhibited the best electrical conductivity properties with the lowest resistivity of 8.68×10⁻² Ω cm, a Hall mobility of 10.9 cm² V⁻¹ s⁻¹ and a carrier concentration of 6.5×10¹⁸ cm⁻³. The films showed excellent transparency with average transmittances of over 85% in the visible range.     

Direct reduction of graphene oxide films into highly conductive and flexible graphene films by hydrohalic acids

We report a simple but highly-effective hydrohalic acid reducing method to reduce graphene oxide (GO) films into highly conductive graphene films without destroying their integrity and flexibility at low temperature based on the nucleophilic substitution reaction. GO films reduced for 1 h at 100 °C in 55% hydroiodic (HI) acid have an electrical conductivity as high as 298 S/cm and a C/O ratio above 12, both of which are much higher than films reduced by other chemical methods. The reduction maintains good integrity and flexibility, and even improves the strength and ductility, of the original GO films. Based on this reducing method, a flexible graphene-based transparent conductive film with a sheet resistance of 1.6 kΩ/sq and 85% transparency was obtained, further verifying the advantage of HI acid reduction.     

Bio-inspired cross-linking with borate for enhancing gas-barrier properties of poly(vinyl alcohol)/graphene oxide composite films

The demand for flexible and transparent barrier films in industries has been increasing. Learning from nature, borate ions were used to cross-link poly(vinyl alcohol) (PVA) and graphene oxide (GO) to produce flexible, transparent high-barrier composite films with a bio-inspired structure. PVA/GO films with only 0.1 wt% GO and 1 wt% cross-linker exhibited an O₂ transmission rate <0.005 cc m⁻² day⁻¹, an O₂ permeability <5.0 × 10⁻²⁰ cm³ cm cm⁻² Pa⁻¹ s⁻¹, and a transmittance at 550 nm >85%; thus, they can be used for flexible electronics. Fourier transform infrared spectrometry and X-ray photoelectron spectroscopy indicated that the outstanding barrier properties are attributed to the formation of chemical cross-linking involving borate ions, GO sheets, and PVA, similar to the borate cross-links in high-order plants. Comparing our experimental data with the Cussler model, we found that the effective aspect ratio was significantly increased after cross-linking, suggesting that cross-linking networks connected GO with each other to form ultra-large impermeable regions. A feasible green technique, with potential for commercial production of barrier films for flexible electronics was presented.     

Ce0.9Gd0.1O1.95 supported La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes for solid oxide fuel cells

Lanthanum-based iron- and cobalt-containing perovskite has a high potential as a cathode material because of its high electro-catalytic activity at a relatively low operating temperature in solid oxide fuel cells (SOFCs) (600–800). To enhance the electro-catalytic reduction of oxidants on La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF), Ga doped ceria (Ce_0.9Gd_0.1O_1.95, GDC) supported LSCF (15LSCF/GDC) is successfully fabricated using an impregnation method with a ratio of 15 wt% LSCF and 85 wt% GDC. The cathodic polarization resistances of 15LSCF/GDC are 0.015 Ω cm², 0.03 Ω cm², 0.11 Ω cm², and 0.37 Ω cm² at 800 °C, 750 °C, 700 °C, and 650 °C, respectively. The simply mixed composite cathode with LSCF and GDC of the same compositions shows 0.05 Ω cm², 0.2 Ω cm², 0.56 Ω cm², and 1.20 Ω cm² at 800 °C, 750 °C, 700 °C, and 650 °C, respectively. The fuel cell performance of the SOFC with 15LSCF/GDC shows maximum power densities of 1.45 W cm^?2, 1.2 W cm^?2, and 0.8 W cm^?2 at 780 °C, 730 °C, and 680 °C, respectively. GDC supported LSCF (15LSCF/GDC) shows a higher fuel cell performance with small compositions of LSCF due to the extension of triple phase boundaries and effective building of an electronic path.     

The effect of annealing on electrical properties of fluorinated amorphous carbon films

Fluorinated amorphous carbon (a–C:F) films have been deposited by electron cyclotron resonance chemical vapor deposition (ECR–CVD) at room temperature using C₄F₈ and CH₄ as precursor gases. The chemical compositions and electrical properties of a–C:F films have been studied by X-ray photoelectron spectroscopy (XPS), capacitance–voltage (C–V) and current-voltage (I–V) measurements. The results show that C–CF_x and C–C species of a–C:F films increase and fluorine content decreases after annealing. The dielectric constant of the annealed a–C:F films increases as a result of enhancement of film density and reduction of electronic polarization. The densities of fixed charges and interface states decrease from 1.6 × 10¹⁰ cm^− 2 and (5–9) × 10¹¹ eV^− 1 cm^− 2 to 3.2 × 10⁹ cm^− 2 and (4–6) × 10¹¹ eV^− 1 cm^− 2 respectively when a–C:F films are annealed at 300 °C. The magnitude of C–V hysteresis decreases due to reduced dangling bonds at the a–C:F/Si interfaces after heat treatment. The conduction of a–C:F films shows ohmic behavior at lower electric fields and is explained by Poole–Frankel (PF) mechanism at higher electric fields. The PF current increases indicative of reduced trap energy when a–C:F films are subjected to higher annealing temperatures.     

Electrical contacts to nitrogen incorporated nanocrystalline diamond films

The effect of surface plasma treatment on the nature of the electrical contact to the nitrogen incorporated nanocrystalline diamond (n-NCD) films is reported. Nitrogen incorporated NCD films were grown in a microwave plasma enhanced chemical vapor deposition (MPECVD) reactor using CH₄ (1%)/N₂ (20%)/Ar (79%) gas chemistry. Raman spectra of the films showed features at ∼ 1140 cm^− 1, 1350 cm^− 1(D-band) and 1560 cm^− 1(G-band) respectively with changes in the bonding configuration of G-band after the plasma treatment. Electrical contacts to both untreated and surface plasma treated films are formed by sputtering and patterning Ti/Au metal electrodes. Ohmic nature of these contacts on the untreated films has changed to non-ohmic type after the hydrogen plasma treatment. The linear current–voltage characteristics could not be obtained even after annealing the contacts. The nature of the electrical contacts to these films depends on the surface conditions and the presence of defects and sp² carbon.     

Structural,electrical and optical properties of molybdenum-doped TiO2 thin films

Molybdenum doped TiO₂ (MTO) thin films were prepared by radio frequency (RF) magnetron sputtering at room temperature and followed by a heat treatment in a reductive atmosphere containing 90% N₂ and 10% H₂. XRD and FESEM were employed to evaluate the microstructure of the MTO films, revealing that the addition of molybdenum enhances the crystallization and increases the grain size of TiO₂ films. The optimal electrical properties of the MTO films were obtained with 3 wt% Mo doping, producing a resistivity of 1.1×10^?3 Ω cm, a carrier density of 9.7×10²⁰ cm^?3 and a mobility of 5.9 cm²/Vs. The refractive index and extinction coefficient of MTO films were also measured as a function of film porosity. The optical band gap of the MTO films ranged from 3.28 to 3.36 eV, which is greater than that of the un-doped TiO₂ film. This blue shift of approximately 0.14 eV was attributed to the Burstein–Moss effect.     

Ion implantation and anneal to produce low resistance metal–diamond contacts

Contacts to boron-doped, (100)-oriented diamond implanted with Si or with Si and B were formed and the effects of dose, implantation energy and anneal treatment on the specific contact resistance were examined. Ti/Au contacts on heavily implanted diamond (10¹⁶ Si ions cm⁻², E_i=30 keV or 10¹⁷ Si and B ions cm⁻², E_i=15 keV (Si) and E_i=10 keV (B)) had a specific contact resistance lower than the best contacts produced on unimplanted diamond. A specific contact resistance of (1.4±6.4)×10⁻⁷ Ω cm⁻² was achieved following a 450°C anneal. The results were consistent with a reduction in barrier height brought about by silicide formation. Light silicon implantation (10¹³ ions cm⁻²) or relatively light dual implantation (B, Si<10¹⁶ ions cm⁻²) did not reduce the specific contact resistance. Increasing the diamond conductivity by 4×10⁴ decreased the specific contact resistance by over three orders of magnitude, in agreement with the trend observed by Prins (J.F. Prins, J. Phys. D 22 (1989) 1562).     

Electrochemical and microstructural characteristics of nanoperovskite oxides Ba0.2Sr0.8Co0.8Fe0.2O3−δ (BSCF) for solid oxide fuel cells

Nanoperovskite oxides, Ba_0.2Sr_0.8Co_0.8Fe_0.2O_3?δ (BSCF), were synthesized via the co-precipitation method using Ba, Sr, Co, and Fe nitrates as precursors. Next, half cells were fabricated by painting BSCF thin film on Sm_0.2Ce_0.8O_x (samarium doped ceria, SDC) electrolyte pellets. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS) measurements were carried out on the BSCF powders and pellets obtained after sintering at 900 °C. Investigations revealed that single-phase perovskites with cubic structure was obtained in this study. The impedance spectra for BSCF/SDC/BSCF cells were measured to obtain the interfacial area specific resistances (ASR) at several operating temperatures. The lowest values of ASR were found to be 0.19 Ω cm², 0.14 Ω cm² 0.10 cm², 0.09 Ω cm² and 0.07 Ω cm² at operating temperatures of 600 °C, 650 °C, 700 °C, 750 °C and 800 °C, respectively. The highest conductivity was found for cells sintered at 900 °C with an electrical conductivity of 153 S cm^?1 in air at operating temperature of 700 °C.     

Dose and wavelength dependent study of graphene oxide photoreduction with VUV Synchrotron radiation

Photoreduction has so far received less attention than other approaches used to reduce graphene oxide (GO) like thermal and chemical reduction, although its potential is huge. The mechanism of deoxygenation is still questioned. In photolithographic applications, this means that one cannot predict whether the minimum feature size is close to the diffraction limit or larger. In this work we have studied GO photoreduction with vacuum ultraviolet Synchrotron radiation. GO has been exposed to extreme-UV and soft-X-ray radiation and the ratio between C–O and sp² C–C bonds has been measured by photoemission spectroscopy as a function of dose and number of photons. The deoxygenation rate has been demonstrated to be proportional to the imaginary part f₂ of the oxygen scattering factor. Experiments at the wavelengths where the f₂ of oxygen is close to its maximum (namely close to 2 nm and between 20 nm and 60 nm) have demonstrated that GO can be rapidly reduced (even few minutes exposure time) with doses well below those needed for photothermal heating (the corresponding temperature rise is ∼1 μK) and without damaging the basal plane (the in-plane sp² C–C bonds are not broken). Efficient nanometer scale GO photopatterning is therefore demonstrated to be possible.     

Characteristics of highly (001) oriented (K,Na)NbO3 films grown on LaNiO3 bottom electrodes by RF magnetron sputtering

(K,Na)NbO₃ ferroelectric films were grown on LaNiO₃ coated silicon substrates by RF magnetron sputtering. The conductive LaNiO₃ films acted as seed layers and induced the highly (001) oriented perovskite (K,Na)NbO₃ films. Such films exhibit saturated hysteresis loops and have a remnant polarization (2P_r) of 23 μC/cm², and coercive field (2E_c) of 139 kV/cm. The films showed a fatigue-free behavior up to 10⁹ switching cycles. A high tunability of 65.7% (@300 kV/cm) was obtained in the films. The leakage current density of the films is about 6.0×10^?8 A/cm² at an electric field of 50 kV/cm.